doc: Adding part 2 of the getting started tutorial

  o doc/examples/running-commands: New example project of a `manual` build element

  o doc/sessions/running-commands.run: New session file to capture bst output

  o doc/source/sessions-stored: Added new generated sessions

  o doc/source/tutorial/running-commands.rst: New tutorial entry describing how
                                              commands are run in the sandbox

  o tests/examples/running-commands.py: Test case validating the tutorial's assertions